Method and apparatus for bending wire



March 26,- 1963 s. A., PLATT 3,032,797

METHQD AND APPARATUS FOR BENDING WIRE Filed Nov. 7. 1958 '7 Sheets-Sheet1 i E I m m -T 2 1 g f A i H my INVENTOR.

STEPHEN A. PLATT L BY M M March 26, 1963 s. A. PLATT 3,082,797

METHOD AND APPARATUS FOR BENDING WIRE Filed Nov. 7, 1958 v 7Sheets-Sheet 2 INVENTOR. STEPHEN A. PLATT ATTORNEYS March 26, 1963 s. A.PLATT 3,032,797

METHOD AND APPARATUS FOR BENDING WIRE Filed Nov. 7, 1958 '7 Sheets-Sheet3 INVENTOR. STEPHEN A. PLATT A TTORNE Y5 March 26, 1963 s. A. PLATTMETHOD AND APPARATUS FOR BENDING WIRE 7 Sheets-Sheet 4 Filed NOV. 7,1958 .En m lmmu &

INVENTOR. STEPHEN A. PLATT March 26, 1963 s. A. PLATT 3,082,797

METHOD AND APPARATUS FOR BENDING WIRE Filed Nov. 7, 1958 7 Sheets-Sheet6 INVENTOR. 5O STEPHEN A. PLATT A T TORNE Y5 March 1963 s. A. PLATT3,082,797

METHOD AND APPARATUS FOR BENDING WIRE Filed Nov. 7, 1958 7 Sheets-Sheet'7 wrm:

INVENTOR. STEPHEN PLA 7' 50 BY M ATTORNEYS United States Patent3,082,797 lfvlETHOD AND APPARATUS FQR BENDING WIRE tephen A. Piatt, 1169Fulton St, Grand Haven, Mich. Fiied Nov. '7, 1958, Sea. No. 772,433 4@iairns. (Cl. 140-71) This invention relates to ways and means forbending wire. More particularly, it relates to a method and apparatusfor bending wire to be used as electrical resistance, heating elementsand the like.

Sinusoidally bent, wire resistance elements are used in a number ofappliances, such as heating pads, grills and the like. Consequently,there is a demand for sinusoidally bent resistance wire and machineshave been developed to bend wire to supply this demand. An example ofsuch a machine is described in US. Patent No. 2,456,353 to Wolf, et al.

The machines heretofore developed have a disadvantage however, in thatfor a given width of resistance wire element produced thereby the amountof wire per unit ength of element is less than that desired. In short,the

amount of wire per unit area of element is unduly limited. Thesemachines bend the resistance wire in substantially perfect sine waves.Even the most efiicient of these machines cannot form the wire closerthan with parallel traverse wires or bars. Because the minimum radius ofbend of resistance wire is limited by the physical properties of thewire, once the minimum bending radius has been reached, the quantum ofwire per unit area of element cannot be increased in the case of suchmachines without damaging the wire. On the other hand, the demand of theart is for an increased thermal output per per unit area of wire elementand from economic and engineering considerations, the optimum design ofa bent resistance wire element calls for an increase in the amount ofwire per unit area of element. This the prior art machines are incapableof delivering.

The machines heretofore developed produce sinuated element withsubstantially parallel bars, whether the return bends be very small orvery large. Where the sinuated element can be taken direct from themachine and fabricated into its final sheath without intermediatehandling, no trouble is experienced. However, large volume manufacturehas made it necessary .to store finished element in continuous lengthson large reels, in large quantity, between machine and assembly lines.This calls for sinuated element with its turns touching each other toprevent interlacing of one element with another when they slip on eachother, or when one element falls next to another on the productiontable.

Another problem that had to be met was that of crowding all the bars ofsinuation per running inch or foot possible, both for maximum storageper given reel and for anti-tangling characteristics.

A general object of this invention is to develop ways and means forproducing flat sinuated wire elements wherein the amount of resistancewire per unit area of element :is increased. More particularly, it is anobject of this invention to provide ways and means for bendingresistance wire wherein the only inherent limitation is that of the wireitself, namely, the minimum bending radius that the wire can withstandwithout fracturing or cracking.

Another object of this invention is a machine which produces therequired element form. Extremes are now required in industry rangingfrom bends as short as one wire diameter up to A; inch radius. It willbe seen why physical handling of such requires turns to be touchinguntil just prior to stretching into sheath assembly.

A specific object of this invention is to provide a method for bendingresistance wire into an element hav- "ice ing the maximum amount of wireper unit area of element.

Another specific object of this invention is to provide an apparatus forcontinuously and controllably bending resistance wire into an elementhaving the maximum amount of wire per unit area of element.

These and other objects which may appear as this specification proceedsare achieved by this invention which shall be described in the contextof the drawings wherein:

FIG. 1 is a front elevation of a preferred embodiment of the machine ofthis invention;

FIG. 2 is an enlarged, front, elevational view of a portion of theapparatus of FIG. 1 with a portion of the apparatus broken away toreveal interior structure;

FIG. 3 is a partial, side elevation view taken along the lines IIIIII ofFIG. 1 with portions of the supporting structure of the apparatus brokenaway to reveal internal structure;

FIG. 4 is a fragmentary, sectional, partially broken plan view of theapparatus taken along the lines IVIV of FIG. 1;

FIG. 4a is an enlarged, plan view of the mandrels shown in FIG. 4;

FIG. 5 is a sectional, plan view taken along the lines VV of PEG. 2 andshowing a portion of the bent wire travel resist means of the apparatusof FIG. 1;

FIG. 6 is a plan view of the apparatus of FIG. 1 which view has beensomewhat reduced in size; and

FIGS. 7-14 are diagrammatic views illustrating progressively therelative movements of the wire feeding head and mandrels during oneoperative cycle of the apparatus of HG. 1.

In general, the drawings show that a basic concept of this invention isbending the wire so that it back tracks or doubles back against itselfin a plane with the result that the marginal terminal loops arepartially overlapped laterally of the Wire.

In terms of method, the drawings broadly disclose the steps of pushingwire under tension in a direction of travel while simultaneously in aplane looping the wire transversely to a line in said direction; firstto one side of said line and then to the other side of said line, andresisting movement of looped wire in said direction of travel. Theadjacent loops of the bent wire elements thus formed will usually be incontact with one another and, unless this condition is corrected, willshort circuit the oppositely disposed loops under use conditions.Consequently, while not shown, the method of this invention comprisesthe step of separating (or breaking contact of) adjacent loops.

in terms of apparatus, the drawings broadly disclose a machine forcontinuously forming bent resistance wire which comprises means forpushing wire under tension in a direction of travel, means forsimultaneously looping in a plane said wire transversely to a line insaid direction, first to one side of said line and then to the otherside of said line, and, means for resisting movement of the looped wirein its direction of travel. The sinuating mechanism has a wire feedinghead adapted to travel in a figure 8 shaped path in a plane. Themechanism also has a pair of reciprocable mandrels oppositely disposedon each side of said line of travel and adapted to move into and out ofsaid plane in coordination with the movement of the Write feeding head.Each mandrel is adapted to move oppositely to the other and to comebetween the wire and the wire feeding head as said wire feeding headreverses its movement parallel to the direction of trvael of thefinished wire, and to withdraw from said plane as said wire feeding headmoves towards it. The movement of the feeding head in wrapping the wirearound each of the mandrels pushes the sinuated or looped wire throughthe 3 machine. This produces the tight loop formation char acteristic ofthe product of this machine.

Structure In somewhat greater detail, it will be observed that thedrawings illustrate a wire bending apparatus 20 into which resistancewire 21 under tension is fed and out of which a continuous, looped wire22 emerges (FIG. 4.) The apparatus comprises a stationary table 23having a fiat, horizontal top surface. A pair of wire bending posts ormandrels 24 and 26 extend vertically and movably through the table inslots 25 and 27, respectively. It will be observed that the mandrels 24and 26 are oppositely disposed and are adapted by means to be laterdescribed herein to reciprocate oppositely and vertically from aposition flush with, or below, the top surface of said table 23 to aposition above the plane of said surface. (The line or direction oftravel of wire through the machine is normal to a line between thesemandrels 24 and 26.

Adjacent the table 23 and opposite said reciprocable mandrels is amovable wire feeding means or head 28 comprising a horizontally disposedbottom plate 29, the top surface of which is disposed in the same planeas that of the top surface of the table 23 (FIGS. 4 and 6). Adjacent thetop of bottom plate 29 and facing one another is a pair of curved guidesurfaces formed by the circumferential periphery of a horizontallydisposed, preferably rotatable, disk 36 on an axial shaft 31 and by thecircumferential periphery of another horizontally disposed, preferablyrotatable, disk 32 on an axial shaft 33. The adjacent portions of theguide surfaces are spaced apart a distance slightly more than theoutside diameter of the wire 21. Above and adjacent to said guidesurfaces is a retention plate 34 attached to said bottom plate 29 as byscrews 35.

Below and attached as by rivets to said bottom plate is a horizontallydisposed guide plate 47 (FIG. 3). The front portion of the bottom plate(with reference to direction of travel of the unbent wire 21) isslidable in a transversely arranged track 49 between a stationary frontsup port block 36 and a retaining plate 37 attached to the block. Theback portion of said guide plate (again with reference to the directionof travel of the unbent wire 21) curves downwardly in sliding engagementwith a movable drive rod 38. Secured to the underside of said guideplate 47 is one flange of an angle iron 39. The other flange of theangle iron 39 is in sliding contact with said drive rod 38 and inconjunction with the curved portion of the guide plate maintains thewire feeder 28 in position on said drive rod. 1 i

The drive rod 38 is pivotally secured to the ends of a pair of rockerarms 40 and 44 by pins 41 and 45, respectively (FIG. 6). The other endsof the rocker arms 40 and 44 are pivotally attached to the front supportblock 36 by pins 42 and 46 respectively. The rocker arms 40 and 44 arespaced axially of the rod 38. Both rocker arms, it will be noted, lie inthe same plane and are transversely disposed to both the stationaryfront block and the direction of travel of said unbent wire 21 throughthe apparatus.

At the right hand end of the drive rod 38 there is mounted in thehorizontal plane a roller follower assembly 48 with the axis of theroller being vertically disposed. The roller is shown in contact withthe periphery of a disk cam 50 mounted on an axial shaft 51 the axis ofwhich is likewise vertically disposed. The periphery of the disk cam 50comprises a pair of removable cam follower tracks 52 and 54 which haveinwardly curved leading and trailing edges.

At the right hand end of the drive rod 38 there is attached a springpost 56 (FIGS. 1 and 3). The post 56 mounts one end of a helical,tension spring 57. The other end of the spring 57 is attached to thestationary support structure 53 of the machine and biases the drive rodtoward the disk cam 56.

The drive rod 38 in conjunction with the rocker arms 49 and 44, the camfollower assembly and the spring 57, functions to impart reciprocatingbackward and forward motion to the wire feeder means 28.

Transverse motion of the wire feeder means 28 is obtained by transversemotion assembly comprising a pitman (FIG. .6), one end of which ispivotally secured to the bottom plate 29 of the feeder means 28 as by ashaft pin 61. The other end of the pitrnan 60 has a head block 62. Thehead block 62 is secured to an anchor plate as by the bolt 63 and nut67. The anchor plate 65 is seated in the channel 64 of the disk cam 50for radial adjustment and detachably secured by the set screws 66.

Each of the vertically movable mandrels 24 and 26 functions as wirebending posts. The portions thereof which rise above the plane of thetop surface of the table 23 are provided with outer lateral, curved,wire bending surfaces 24a and 26a (FIG. 4a) each of which has a radiusof revolution corresponding to the desired radius of curvature of theloops of the sinuated wire product. In general, this radius is selectedto correspond to the minimum radius at which the particular Wire beingsinuated can be bent Wtihout rupture. The axis of each surface runslongitudinally through the corresponding mandrel.

Each of the vertically movable mandrels 24 and 25 is driven by a springand cam assembly (FIGS. 2 and 3). Thus, with reference to the right handmandrel 24 the bottom end thereof is associated with a cam follower 69,a pivot bar 107 rotatably secured to a stationary shaft 103 mounted inthe stationary support structure 58 and a compression spring 68 attachedto the overhead portion of the structure 58. Similarly, the bottom endof the left hand mandrel 26 is associated with a cam follower 71, apivot bar 109 rotatably secured to the shaft 108 and a compressionspring 70 attached to the overhead portion of the supporting structure58. Both cam followers ride on a rotatable, horizontal radial cam 72.The cam 72 is mounted on and rotated by the rotatable shaft 75. The cam72 has two adjacent cam surfaces, each of which has a constant radiusfor about 270 and a larger radius for about 90 about the axis of the camwhereby each cam surface comprises a lower peripheral portion 73 and araised peripheral portion 74. The raised portion of the one cam surfaceis disposed 180 about the cam axis from the raised portion of the secondcam surface and the radius of each raised cam surface is sufficient toproject the respective mandrels 24 and 26 above the plane of the topsurface of the table 23 when said cam surfaces contact the respectivecam follower members of the mandrels 24 and 26. The bias of the springs68 and 70 toward the cam 72 assures that the cam followers 69 and 70will be in constant contact with the respective cam surfaces.

The drive shaft 75 associated with the radial cam 72 is connected to thegear box 76. By means of the gear box '76 it also drives the disk cam50, thus synchronizing the operation of said mandrels 24 and 26 and thecam 50. The drive shaft 75 is driven from a prime mover (not shown) bymeans of the pulley 77.

Under operative conditions the wire feeder means 28 makes substantiallya transverse figure 8 motion. Accordingly, the mandrels 24 and 26 arecentered in and forgn the ends of the loops in the wire as the wire issinuate The means for resisting movement of looped Wire in the directionof travel thereof comprises an assembly situated above the table 23behind the mandrels (FIG. 2). This assembly comprises an invertedU-shaped frame 81 which carries a set of rubber disks 82 rotatable on ashaft 83. The shaft 83 is arranged transversely to the direction of wiretravel and inserted into an aperture in each of the depending legs ofthe frame 81. The rubber wheels are secured axially of the shaft 83 by apair of clamping bars 84. The clamping bars are axially adjustable alongthe shaft 83 to regulate the degree of resistance to rotation as a groupor with respect to each other. They are clamped to an upper horizontallydisposed suspension rod 85 by thumbscrews 86. The ends of the suspensionrod are mounted to the depending legs of the frame 81. The cross arm 87of the frame 81, disposed under, and adjacent to, a crossbar member '88has a pair of upstanding guide pins 89 passing through the arcuate guidepin slots 90 in the crossbar member 88 ('FIG. 5). The crossbar has athumbscrew assembly 91 for adjusting and setting the angular position ofthe frame 81 with relation to the work traveling under the rubber wheels82.

The crossbar 88 is joined as by machine screws to the depending arms ofvertically movable yoke 93, the crossbar of which is threadedlysuspended on a vertically disposed adjustment stud 95. The upper end ofthe adjustment stud 95 passes through an overhead support frame 94,anchored to said bed block 23. A lock nut 96 threadedly attached to saidthumbscrew 95 is disposed adjacent to and on top of the crossarrn of theupper U-bar 93.

The preferred embodiment of this invention also has a spring adjustmentassembly for the drive rod 38 (FIG. 6). This assembly has a horizontallydisposed shim bar 97, one end of which is interposed between the springpost 56 of the drive rod and the edge of .the bed block 23. The bias ofthe spring 57 causes the spring post to bear against the shim bar 97when the rod 48 is at rest. A short distance from said end and on therocker arm side of said shim bar, the bar is in contact with an arcuatefulcrum 98. The other end of the shim bar 97 is biased away from the bedblock 23 by a spring 100. Byreason of position of the fulcrum 98, theshim is urged against the spring post. The end of the shim bar 97,biased outwardly by the spring 100, is urged inwardly against the springby a thumbscrew 1G1 threadedly passing through an aperture in thevertical flange of an angle iron 102 (attached to the bed block 23). Toassure retention of setting, the thumbscrew has a lock nut 103. Byadjustment of the thumbscrew the axial travel of the rod '38 and therebythe fore and aft movement of the feeding head 28 may be controlled.

Also included may be means for manually moving the wire feeder means 28to facilitate threading wire through the machine at start-up.

It may also be observed that the underside of the disk cam 50 isassociated with a belt drive pulley 104 (FIG. 1) which is also keyed tothe drive shaft 51. Under operative conditions this pulley may be usedto drive the sinuated wire wind-up reel.

Other auxiliary structural features may also be provided, such as, forexample, as shown in FIG. 6, a pair of oppositely disposed, adjustableguide plates 106 which function to guide the looped wire 22 from themachine. They also resist movement of said looped wire 22 from themandrels.

Operation Under operative conditions resistance wire 21, as from asupply reel 116 (FIG. 4), passes through a wire tensioning apparatus112. The wire tensioning apparatus 112 is a conventional device and istherefore not described herein. The apparatus 112 functions to establishand maintain the wire 21 at a constant and uniform tension as it ispulled into the machine 2%.

In the machine 26 the wire 21 is pulled into the wire feeding head 28between the guide surfaces 30 and 32 by the movement of the head 28. Thefeeding head 28 moves in the plane of said table 23 top surface in afigure 8 path transversely to the direction of travel of said wire 21while the mandrels 24 and 26 alternately rise above and withdraw to atleast said plane. The movement of the feeding means and the mandrels 24and 26 are so coordinated that wire emerging from said feeding head iscontinuously looped around the mandrels, first on one side and then onthe other, and previously formed loops are pushed in the direction oftravel against the resistance of the disks 82 whereby the back track inthe looped wire is retained and in some cases may even be increased.

The coordination of these movements may be seen in the context of FIGS.7l4. Thus, as shown in FIG. 7, as the wire feeder means 28 moves leftfrom its extreme right hand position, the incoming wire 21 is positionedbehind the elevated, right hand mandrel 24. As the wire feeder means 28continues its travel to the left, the incoming wire 21 is drawn firmlyabout the right hand mandrel 24 by the tension applied to the wire as itis fed to the wire feeder 28. When the wire has been firmly wrappedabout the mandrel 24, the left hand mandrel 26 withdraws to the topsurface of the table 23 as in FIG. 8. As the right hand wiping disk 30moves past the mandrel 24, it wipes the wire firmly against the surfaceof revolution of the mandrel, producing a loop having a radiuscorresponding exactly to that of the surface of revolution.

As its leftward movement continues, the feeder 28 moves also toward thedirection of travel (backward) as in FIG. 9, thereby leading the wirebehind the lowered left hand mandrel 26 and pushing the previouslyformed loop in the direction of travel. As the wire feeding means 28approaches the leftward limit of its travel, it commences to moveforwardly and the left hand wire mandrel 26 elevates above the level ofthe table 23 and blocks forward movement of the left hand portion of thewire, as in FIG. 10. Upon reaching the left hand limit of its travel,the feeder means 28 starts to move toward the right hand side of theline of travel whereupon the incoming wire 21 is engaged by the guidesurface 32 and is led along therewith in the right hand direction,thereby forming a tight loop about the post 26, as in FIG. 11. As thefeeder means continues its rightward movement, the right hand mandrel 24withdraws to the level of the table 23 as in FIG. 12, and the wirefeeder 28 moves rearwardly as in FIG. 13, pushing the preceding loop outof the way and leading the wire 21 behind the lowered mandrel 24. As thewire feeder 28 completes its travel to the right, the right hand mandrel24 again rises above the level of the table 23 and the feeder 28 movesforwardly. As can be seen, the movement of the wire feeding means 28 ina continuous figure 8 motion in relationship to the continuous elevationand withdrawal movement of the mandrels 24 and 26 results in formationof tight wire loops.

The bent wire element 22 thus formed is pushed by the action of the wirefeeder means 28 under the rubber wheels 82 of the movement resistanceassembly 80. The various thumbscrews of this assembly are adjusted toprovide sufiicient bearing and thus resistance to forward travel ofelement 22. This causes adjacent loops to come together as they arepushed rearwardly without decreasing the radius of the loops below thecritical minimum at which fracturing occurs.

The width W (FIG. 4) of the bent wire elements 22 can be adjusted bychanging the length of the stroke of the transverse motion drive rod 60.This can be accomplished by loosening the lock nut on the shaft 63 andsliding the same between the retaining plates 65 toward the drive shaft51 to decrease the width and away from the drive shaft to increase thewidth. The spacing of the mandrels 24 and 26 is correspondinglyadjusted.

The radius of each loop can be adjusted by substituting thicker orthinner mandrels 24 and 26 and thus a larger or smaller radius aboutwhich the wire is formed. The removable follower tracks 52 and 54 can bechanged to ones of increased radial thickness for larger loops and toones of decreased radial thickness for smaller loops. The angle of theframe 81 in relation to the sinuated wire is set to cause the sinuatedwire to move rearwardly in a straight line rather than curving to eitherside.

The concepts of this invention have a number of advantages. In the firstplace, resistance Wire elements pro duced according to the teachingshereof have almost twice as much wire per unit length for a given widthof element. Second, uniform and consistent loops are made. Third, theonly limiting factor in the curvature of the loops is the wire itself,the radius at which the wire cracks or fractures. Fourth, the curvaturesare consistently accurate. Finally, the machine embodiment of thisinvention can be readily adjusted to different production requirementswith a minimum of effort.

Pushing the formed wire away from the mandrels 24 and 26 produces thetightest possible loop configuration with the loops along opposite sidesbeing arranged in partially overlapping relationship rather than spacedalternate relationship. It is this that attains the objective of packingmore resistance into a unit area of formed wire than was heretoforepossible. This overlapping arrangement must be effected in the initialsinuation of the wire. It cannot be accomplished by attempting to closethe loops once they have been formed in an open configuration.

Other features, advantages and embodiments of this invention will becomeapparent to those in the exercise of ordinary skill in the art in lightof the preceding description. Therefore, it should be noted that as thisinvention may be embodied in several forms without departing from thespirit thereof, the present embodiment is therefore illustrative and notrestrictive, since the scope of the invention is defined by the appendedclaims and all changes that fall within the metes and bounds of theclaims or that form their functional as well as conjointly cooperativeequivalents are therefore intended to be embraced by those claims.

I claim:

1. An apparatus for continuously forming a wide, flat, elongated band oflooped resistance wire, which comprises: wire forming means for pullingwire under constant and uniform tension from a source; said wire formingmeans simultaneously forming loops in said wire with said loops lying ina flat plane and constituting said band, said loops extendingtransversely of and being alternately arranged with respect to thelongitudinal center line of said band and the closed ends thereofforming the margins of said band, said wire forming means also movingsaid band away from said source; and means for resisting movement ofsaid band away from said source and said wire forming means for urgingthe loops at the margins of said band into abutting relationship; saidresisting means having a rubber-like wheel rotatably supported forrotation about an axis extending generally parallel to said loops ofsaid band; a support for said resisting means, said resisting meansbeing movable on said support toward and away from said band andangularly variable with relation to the direction of movement of saidband.

2. An apparatus as recited in claim 1 wherein said rubber-like wheelconsists of a plurality of separate disklike members arranged inside-by-side relationship along said axis; said disk-like members eachbeing rotatable with respect to the others to provide a limiteddifferential action for said resisting means.

3. An apparatus as recited in claim 1 wherein said wirc forming meanshas a head member; a pair of disks rotatably mounted on said headmember, said disks being rotatable about axes normal to said plane ofsaid band and spaced apart only sufliciently to form a passage formovement of wire therebetween; a guide supporting said head forreciprocating movement laterally of said band; driving means secured tosaid guide for shifting said guide and said head parallel to thedirection of movement of said band.

4. An apparatus for continuously forming a wide, fiat, elongated band oflooped resistance wire, which comprises: Wire forming means for pullingwire under constant and uniform tension from a source; said wire formingmeans simultaneously forming loops in said wire with said loops lying ina flat plane and constituting said band, said loops extendingtransversely of and being alternately arranged with respect to thelongitudinal center line of said band and the closed ends thereofforming the margins of said band, said wire forming means also movingsaid band away from said source; said wire forming means having a headmember; a pair of disks rotatably mounted on said head member, saiddisks being rotatable about axes normal to said plane of said band andspaced apart only sufficiently to form a passage for movement of wiretherebetween; a guide supporting said head for sliding reciprocatingmovement laterally of said band; driving means secured to said guide forshifting said guide and said head parallel to the direction of movementof said band; and means for resisting movement of said band away fromsaid wire forming means.

References Cited in the file of this patent UNITED STATES PATENTS905,130 Ayer Dec. 1, 1908 1,273,020 Winsor July 16, 1918 1,439,411Griner Dec. 19, 1922 2,047,717 Van Dresser et a1 July 14, 1936 2,061,579Huyett Nov. 24, 1936 2,188,407 Horton Jan. 30, 1940 2,709,462 Schade May31, 1955 2,747,619 Buttner May 29, 1956 2,898,949 Huszar Aug. 11, 1959

1. AN APPARATUS FOR CONTINUOUSLY FORMING A WIDE, FLAT, ELONGATED BAND OF LOOPED RESISTANCE WIRE, WHICH COMPRISES: WIRE FORMING MEANS FOR PULLING WIRE UNDER CONSTANT AND UNIFORM TENSION FROM A SOURCE; SAID WIRE FORMING MEANS SIMULTANEOUSLY FORMING LOOPS IN SAID WIRE WITH SAID LOOPS LYING IN A FLAT PLANE AND CONSTITUTING SAID BAND, SAID LOOPS EXTENDING TRANSVERSELY OF AND BEING ALTERNATELY ARRANGED WITH RESPECT TO THE LONGITUDINAL CENTER LINE OF SAID BAND AND THE CLOSED ENDS THEREOF FORMING THE MARGINS OF SAID BAND, SAID WIRE FORMING MEANS ALSO MOVING SAID BAND AWAY FROM SAID SOURCE; AND MEANS FOR RESISITING MOVEMENT OF SAID BAND AWAY FROM SAID SOURCE AND SAID WIRE FORMING MEANS FOR URGING THE LOOPS AT THE MARGINS OF SAID BAND INTO ABUTTING RELATIONSHIP; SAID RESISTING MEANS HAVING A RUBBER-LIKE WHEEL ROTATABLY SUPPORTED FOR ROTATION ABOUT AN AXIS EXTENDING GENERALLY PARALLEL TO SAID LOOPS OF SAID BAND; A SUPPORT FOR SAID RESISTING MEANS, SAID RESISTING MEANS BEING MOVABLE ON SAID SUPPORT TOWARD AND AWAY FROM SAID BAND AND ANGULARLY VARIABLE WITH RELATION TO THE DIRECTION OF MOVEMENT OF SAID BAND. 